Various prior art teachings are directed to the separation of acid gases, and more specifically, separations of carbon dioxide from hydrocarbons including methane and lower alkanes. These methods typically utilize high levels of refrigeration to perform condensing duty in absorption and distillation columns effecting the appropriate separations and/or utilize lower molecular weight extractants, such as C.sub.4 to C.sub.6 alkanes to overcome various problems of relative volatility, azeotrope formation and triple point approach. Additionally, certain processes exist which require little or no refrigeration, but produce the carbon dioxide product at low pressure and require high levels of recompression energy to return the carbon dioxide to feed pressure.
One typical mode of effecting a carbon dioxide and lower alkane separation is refrigerated distillation which uses a carbon dioxide-rich liquid reflux from an overhead condenser to wash hydrocarbons from a carbon dioxide-containing stream. The refrigeration power for the overhead condenser becomes excessive at high propane recovery such that, for a high carbon dioxide feed containing about 1-2% propane, it is uneconomical to recover more than about 30% of the propane.
It is further known to use an activated MDEA (methyldiethanolamine) solvent in an absorption/stripping process to perform the specified separation. Carbon dioxide is absorbed from the gas stream at feed pressure, then the carbon dioxide-rich solution is dropped in pressure and in some cases, heated to liberate the carbon dioxide. The process is capable of very high hydrocarbon recoveries, but high capital and operating costs are incurred in recompressing the product carbon dioxide.
Additionally, it is known to utilize various membrane processes, wherein the process exploits the ability of certain materials to preferentially permeate carbon dioxide from carbon dioxide-hydrocarbon mixtures. The technique is capable of achieving high hydrocarbon recoveries. However, the carbon dioxide permeates the membrane at low pressure and must be recompressed. Again, this recompression is both capital and energy intensive.
Finally, it is known to use extractive distillation, such as is typically referred to as Ryan/Holmes processes. The extractive distillation involves addition of the recycled C.sub.4 -C.sub.6 liquid alkane to the distillation column at a location above the feed. The extractive agent improves the relative volatility of carbon dioxide to propane and allows high recoveries to propane and higher alkanes. Approximately equivalent amounts of power and heat are required by the process when the recommended ratio of 10 moles of liquid agent per 100 moles of feed is employed. Small increases in the amount of solvent may decrease the power input somewhat, but the process becomes increasingly less efficient, and total energy needs increase substantially if solvent circulation is increased to a greater degree.
Relevant prior art includes U.S. Pat. No. 4,428,759 wherein in FIG. 2 a three column configuration of extractive distillation is performed of a feed stream comprising methane, ethane, nitrogen and carbon dioxide. The carbon dioxide, however, is removed as the bottom stream from the initial column, while nitrogen and methane are removed as an overhead through a condenser where the extractive agent is added. In the second column, additional extractive agent is added to such secondary distillation wherein a methane product is recovered as an overhead, and the carbon dioxide is removed to yet another or third column for separation from the C.sub.4+ material in a distillation column, which is run by an overhead condenser and a bottom reboiler. FIG. 3 shows a two column extractive distillation of CO.sub.2 and C.sub.3.
In U.S. Pat. No. 4,293,322 a carbon dioxide-hydrogen sulfide separation is disclosed using an extractant of C.sub.3 -C.sub.6 alkanes wherein in the initial column, carbon dioxide is removed as an overhead fraction and the extractant and the hydrogen sulfide removed as a bottom fraction for subsequent further separation in a black box configuration whereby the hydrogen sulfide is segregated from the extractant, which in turn is recycled to the initial distillation column.
U.S. Pat. No. 4,254,094 discloses the separation of synthesis gas-type feed strams into a hydrogen product, a carbon dioxide vent stream and a hydrogen sulfide by-product whereby in column 24 a carbon dioxide-loaded physical solvent is utilized to strip hydrogen sulfide without co-absorption of carbon dioxide contained in the feed to that column. Carbon dioxide is absorbed in the stage of the column directly above the hydrogen sulfide column, specifically the upper stage 25 of the overall column 23.
Additional art of less relevance, but disclosing light gas separations and multicolumn extractive distillation includes; U.S. Pat. Nos. 4,318,723, 4,370,156, 4,383,842 and 4,462,814.
None of the above-recited prior art disclosures provide an efficient, heat-based process to separate carbon dioxide from propane and lower molecular weight natural gas liquids. Furthermore, it is readily apparent from the literature that the need for such a process to economically integrate with the existing energy sources where the separation is to occur, has not been addressed. Thus, the potential for economic savings by using the low cost energy source contemplated from the present invention, has not yet been realized.